1. Technical Field
This invention relates generally to manufacturing and shaping of parts constituted of reasonably rigid material, and more particularly to rapid pattern formations by sectioned object machining which facilitates casting of such pattern.
2. Discussion of the Prior Art
It is becoming paramount that casting patterns and their associated tools be fabricate faster in free-form, at lower cost, have superior surface finish and require little or no remachining. Although there is no technology that is directed solely to rapid making of consumable patterns the technology that is relevant to these objectives fall into generally four categories: extruding, spraying, curing and laminate forming. Each has significant disadvantages that fail to meet all of the objectives.
Extrusion uses a heated nozzle to extrude hot wax or other polymer filaments, such as nylon, into a shape. It is limited to patterning of simple shapes; it is fragile, subject to delamination, and is expensive.
Spraying can be either (i) an emulsion type where a binder is sprayed onto a metal powder which is subsequently laser sintered, or (ii) thermally sprayed at Spraying is expensive, slow and produces a mediocre surface finish which is porous.
Curing, oftentimes referred to as sterolitography, involves subjecting photo-sensitive polymer resins in thin layers to a laser beam for curing and thence repeating such steps to build up a part or pattern. It is generally slow, useful only for nonmetals and tooling, and requires expensive emission control measures because of the odorous materials. It also demands post machining because the surface is not stable during storage, is somewhat brittle, and is size limited.
Prior art laminate forming involves cutting of metal plies, paper, plastic sheets or foam, by use of a laser beam, hot wire or mechanical cutter to make sheet like cutouts; the cutouts are subsequently assembled to form a unit. Laminate forming first was carried out without a computer graphic guide, the cutter following a mechanical guide. More recently, the prior art has programmed the cutter to follow a computer model or graphic, such as shown in U.S. Pat. No. 4,675,825. The latter type of laminate forming has been used to shape metal but fails to eliminate post-machining of the unit, lacks high structural integrity because of the need to use very thin laminates, is incapable of making complex parts such as intricate engine cylinder heads, manifolds or engine blocks, and is slow due to the high number of handling and finishing operations resulting from the number of laminates.
The requirement of post-machining for state of the art laminate forming is a significant drawback. Machining used for this process works only to cut edges of the thin plies or sheet metal (up to 12 mm) inhibiting full contour machining. When the sheets are assembled, the total surface will be somewhat ragged or stepped, requiring post-machining to obtain acceptable surface finishing. If the laminates are foam, paper or plastic, the same problem remains because of the inability to fully free-form the edges of the material; moreover, foam materials can produce a high degree of porosity in the final surface when assembled.
It is an object of this invention to rapidly make a consumable pattern by a more economical sectioned-object machining technique adaptable to the most complex shape, such pattern being used to form a mold therearound and thence displaced by metal. The method results in a cast product that has a surface finish that is superior, the method having little or no disadvantageous manufacturing side effects.
The invention, in a first aspect, is a method that comprises: (a) designing a three-dimensional fully surfaced computer graphic model of the part; (b) sectioning the graphic model into graphic members which are at least one of blocks and slabs; (c) carving a solid fugitive member for each graphic member that is proportional to and envelopes such graphic member, said carving being carried out by accessing two or more sides of such solid fugitive member to at least essentially duplicate the corresponding graphic member; and (d) securing the carved solid members together to replicate the graphic model as a usable unitary fugitive pattern.
The invention in a second aspect is a method of rapidly making casting comprising: (a) designing a three-dimensional fully surfaced computer graphic model of the casting including complex free-form undercut or substantially hidden interior surfaces; (b) within a computer, subdividing the graphic model into graphic members selected from blocks and slabs; (c) carving a solid non-graphic physical member to replicate each of the graphic members, the physical members being (i) constituted of a meltable or evaporative solid material (i.e. wax or polystyrene), (ii) proportional to and enveloping its corresponding graphic member, and said carving being carried out to at least access, carve into and penetrate two or more sides of each solid physical member to permit such carving to essentially duplicate the corresponding graphic member including all of the interior surfaces; (d) securing the carved solid members together to replicate the graphic model and form a unitary investment pattern; (e) forming a mold around the pattern, and (f) casting metal within the mold while removing the pattern from such mold either during pouring of the molten casting material thereinto or prior thereto by dissolution or melting.
Carving is preferably carried out on opposite sides of each member, such as by CNC milling machines, the thickness of the solid members being in the range of 14-150 mm and each of the solid members having opposed sides which are parallel or skewed with respect to each other.
The methods are capable of being applied to the making of unique machined metal functional parts, to the making of rapid developmental prototypes of cast metal parts, and to the rapid making of initial or preproduction cast components.